1. Field of the Invention
This invention relates to electric volume control circuits for controlling the volume or level of a signal according to the level of a control signal and, more particularly, to electronic volume control circuits which can control the audio signal level in a natural manner according to a control signal.
2. Prior Art
Recent audio and video apparatuses include pluralities of ICs. Many of these ICs are controlled according to digital signals from microcomputers provided in apparatuses. Inside or outside such an IC, a D/A (digital/analog) converter is provided. An electronic volume control circuit effects volume or level control according to a digital signal from such a microcomputer. Such an electronic volume controls the audio signal level according to the level of a control signal.
FIG. 1 shows an electronic volume such as above. An audio input signal is provided from an input terminal 101 on a GCA (gain control amplifier) 102, with the gain thereof varied according to a gain control signal and thereby level controlled. The level controlled signal obtained from the GCA 102 is led to an output terminal 103. The gain of the GCA 102 is controlled according to a digital signal from a microcomputer 104. The digital signal from the microcomputer 104 is a serial signal and is converted in a serial/parallel converter 105 into a parallel signal. The parallel signal is then converted into an analog signal in a D/A converter 106. The analog signal is applied to a variable reference voltage supply 107, which provides a reference voltage varying according to the control signal. According to changes in the reference voltage of the variable reference voltage supply 107, the level of a control signal from a controller 108 is changed to change the gain of the GCA 102.
The relation between the reference voltage of the variable reference voltage supply 107 and the gain of the GCA 102 is shown by the solid curve in FIG. 2. Reducing reference voltage of the variable reference voltage supply 107 increases attenuation of the audio input signal. In FIG. 2, -100 dB is regarded to be zero audio signal level. The staircase waveform shown along the solid curve in FIG. 2 shows steps of the digital signal obtained from the D/A converter 106. Usually, a 5-bit control signal is applied to the D/A converter 106, and a 32-step digital signal is provided.
In the circuit shown in FIG. 1, electronic volume is thus controlled according to the digital signal from the microcomputer.
In this circuit, however, the attenuation is only controlled in 32 steps over a wide range from 0 to -100 dB. In this case, when greatly attenuating the signal level, sufficient steps can not be secured in an attenuation range above about -30 dB in which the change of level is clearly understood by listeners. This means that the volume of sound in the audio apparatus is reduced largely when it is reduced by one step in a high level, and a listener feels departure from the normal sense of perception.
FIG. 3 shows the GCA 102 and the controller 108. In the GCA 102, the audio input signal from the terminal 101 is attenuated according to a gain control signal from terminals 208 and 209 and then led to the output terminal 103. The control circuit 108 includes a transistor 211, the base of which a control signal for attenuation from a terminal 212 is applied, a resistor 214, through which a current corresponding to the emitter voltage on the transistor 211 flows, and a differential amplifier 215, which compares a voltage at a point 0 and a reference voltage V.sub.ref and generates two control voltages for controlling the gain of the GCA 102. To the control signal input terminal 212, the output voltage of the variable reference voltage supply 107 is applied.
When the voltage of the attenuation control signal from the terminal 212 is zero, the transistor 211 is "off", and its emitter current is zero. In this state, a current flows through the resistor 214, providing a predetermined voltage determined by values of resistors 214, 218, and 219 at the point 0. The voltage at the point 0 in this state of the circuit is sufficiently low compared to the reference voltage V.sub.ref.
Thus, transistors 220 and 221 are "on" and "off", respectively, and terminals 208 and 209 are at "H" and "L" levels, respectively.
In GCA 102, transistors 222 and 223 are "on" while transistors 224 and 225 are "off" in dependence on the "H" and "L" levels on the terminals 208 and 209 respectively.
In the meantime, currents corresponding to an audio input signal from the input terminal are generated at the collectors of transistors 226 and 227. Transistors 222 to 225 serve as a current distributor for constant current sources 228 and 229. When the transistors 222 and 223 are "on" while the transistors 224 and 225 are "off", the corrector signal at the transistors 226 and 227 is provided at the same level to the collectors of the transistors 222 and 223. The corrector signal from the transistors 222 and 223 is passed through current mirror circuits 230 to 232 for DC component removal and then led in a double end output form to the output terminal 103. The DC component at the output terminal 103 is superimposed on the reference voltage V.sub.ref.
As shown above, when the control signal voltage is zero, the audio signal is not attenuated.
When the attenuation control signal voltage from the terminal 212 is increased, the transistor 211 is turned on to cause a current corresponding to its emitter current to flow through the resistor 219.
With this emitter current, the voltage at the point 0 is increased. The voltage increase at the point 0 reduces the corrector current in a transistor 220, causing a corrector current in a transistor 221. As a result, the voltage level at the terminal 208 is decreased, and the voltage level at the terminal 209 is increased.
As a result, the corrector current in the transistors 222 and 223 is reduced to reduce the audio input signal transmission rate (percentage) from 100%. In this way, the input audio signal is attenuated.
In the above way, the circuit shown in FIG. 3 can attenuate the input signal according to the control signal.
FIG. 4 shows the attenuation obtainable in the circuit shown in FIG. 3. In the graph, the ordinate is taken for the transmission rate (inverse of attenuation), and the abscissa is taken for the control signal level (=reference voltage).
As shown, the attenuation is increased with increasing control signal level. However, when the control signal level is below V.sub.BE, the transistor 211 is "off", and a certain current is caused therethrough when the level V.sub.BE is exceeded, that is, the attenuation is suddenly increased when the signal level V.sub.BE is exceeded. Therefore, when the volume is quickly reduced, a listener feels a departure from the normal sense of perception.